In recent years, display apparatus such as flat panel displays (FPDs: liquid crystal display apparatus, organic EL display apparatus, and the like) and a solar cell have been made more and more flexible from the viewpoints of, for example, easy conveyance, storing easiness, and design property, and the weight reduction and thinning of the transparent substrate to be used in the display apparatus and the solar cell, and in addition, an improvement in bending property thereof have been requested. Glass substrates have been conventionally used in many cases as transparent substrates for use in the display apparatus and the solar cells. The glass substrate is excellent in transparency, solvent resistance, gas barrier property, heat resistance, and dimensional stability. However, when one attempts to achieve the weight reduction and thinning of a glass material, which forms such glass substrate, the following problem arises. That is, though the glass substrate shows some degree of bending property, the brittleness of glass against a crack or the like is conspicuously exhibited, and hence the glass substrate becomes difficult to handle.
In view of the foregoing, an attempt has been made to use a resin film excellent in stiffness, and having a lighter weight and more excellent bending property than those of glass in a substrate for a display device. However, a resin for constructing the resin film has a higher coefficient of thermal expansion than that of an inorganic material such as the glass, and hence the substrate for a display device obtained by using the resin film involves the following problems. The distortion of the substrate occurs, or a crack occurs in an electrode at the time of the formation of a circuit to cause an increase in resistance value or disconnection. In addition, when a resin having an aromatic ring and/or a heterocycle introduced into its main chain for improving its heat resistance is used, the resin film undergoes coloring and hence a display device having sufficient display performance cannot be obtained. In order that such problems concerning the thermal expansion and the coloring may be resolved, the following has been investigated. A substrate for a display device is obtained by using a resin such as a polycarbonate, a polyarylate, or a polyether sulfone. However, such resin cannot provide a substrate for a display device having sufficient dimensional stability because its coefficient of linear expansion is about ten times as high as that of the glass. Further, the resin film does not have sufficient gas barrier property. Accordingly, when the resin film is used in a substrate for a display device, a gas barrier layer needs to be formed on its surface. As a result, a problem such as an increase in the number of production steps, a reduction in yield, or an increase in cost arises.
To solve those problems, such a substrate that a resin layer is formed on a glass surface has been disclosed (see, for example, Patent Documents 1 and 2). However, it has still been unable to obtain a transparent substrate showing sufficient bending property even with such technology because the technology is insufficient in terms of the optimization of a resin material for reinforcing the glass.